Lecture by Prof. Sense Jan van der Molen, Leiden University

Title: Probing interlayer interaction in Van der Waals materials

AbstractSeveral layered materials can be mechanically exfoliated, down to atomically thin sheets, the most famous example being graphene. This opens the possibility to stack different layer types together to form materials with novel properties: the so-called van der Waals (vdW) systems. Clearly, the extent to which their properties differ from those of the mother materials is governed by the interaction between the different layers. Here, we investigate interlayer interactions using several novel techniques developed in our lab.

At the basis of our research is a low-energy electron microscope (LEEM), called ESCHER. With this unique instrument, we can probe the reflection of electrons with tunable energies 0-30 eV, where 0 eV energy refers to the vacuum level. Not only does this allow for microscopy, with a resolution of 1.5 nm, it also allows us to do local spectroscopy. Moreover, we have recently extended our instrument with a groundbreaking form of transmission electron microscopy (TEM), which probes at extremely low electron energies (0-30 eV range). Hence, we have coined it ‘eV-TEM’.

We apply both LEEM and eV-TEM to graphene layers of varying thickness. In the normal direction, we are able to probe unoccupied states that exist between the graphene layers. With every additional layer of graphene, an interlayer state is added to the system, which can hybridize with the other interlayer states to form quasi-molecular orbitals. In electron reflection, the resulting eigenstates appear as minima in the spectrum R(E). In an electron transmission spectrum T(E), however, they show up as resonant maxima. From both functions, we can quantitatively determine the hybridization energies of the overlapping states and hence follow the formation of new states as new graphene layers are added.

Next, we study the 2D-dispersion relation of these states within the plane. For that, we have developed a novel technique, coined angle-resolved reflected-electron spectroscopy (ARRES) [1]. Applying ARRES, we investigate simple Van der Waals materials consisting of flakes of few-layer graphene and h(exagonal) BN separately, as well as their combination. Interestingly, we find virtually no hybridization between hBN and graphene layers. This substantiates that hBN is an excellent substrate to isolate graphene from its environment over a wide energy range. [2]

Summarizing, we have a unique possibility to investigate band structure formation in a large range of vdW systems. Knowledge on this is crucial to tailor the properties of such layered materials, as they are built up in a LEGO-like fashion.